Torsional vibration damping mechanisms have long been used to reduce the adverse effects of torsional vibrations or fluctuating torques in vehicle drivelines. Such torsional vibrations or fluctuating torques, herein after referred to as torsionals, emanate primarily from engine power pulses and torque spikes, and from abrupt changes in driveline torque due primarily to rapid engine acceleration/deceleration and transmission ratio changes.
Most known, prior art torsional vibration damping mechanisms have employed springs disposed in parallel with a mechanical friction device. Driveline torque is normally transmitted by the springs and flexing of the springs attenuates or reduces the potential amplitude of the driveline torsionals. The mechanical friction device dampens or reduces the rate of spring recoil. When the amplitude of the torsionals is less than the breakaway torque of the friction device, spring flexing does not occur and the torsionals are transmitted without benefit of attenuation.
Effective damping of torsionals by known torsional vibration damping mechanisms has become increasingly more difficult due to current development trends necessitated by a need to improve vehicle efficiency. The need to improve vehicle efficiency has resulted in reductions in vehicle size and weight, reductions in inertia of driveline components such as flywheel masses, reductions in the number of engine cylinders or chambers, reductions in engine speed, increases in the number of transmission gear ratios, reductions in transmission oil viscosity, and increased use of torque converter bypass clutches.
These developments have dramatically increased long existing problems with transmission gear rattle noise, vehicle body noise, and vehicle jerk. Gear rattle is often divided into two classes i.e., idle rattle and in-gear rattle. In-gear rattle is sometimes referred to as driving mode gear rattle. Driveline torsionals provide the excitation for both types of rattle and the rattle or noise occurs when meshed gear teeth of unloaded gears bounce against each other. Body noise or body boom, as it is sometimes referred to, often occurs when an engine is lugged; under such a condition, driveline torsionals cause body components, such as sheet metal panels, to resonate. Vehicle jerk, also known as tip-in/tip-out occurs in response to abrupt engine acceleration/deceleration and ratio changes.
The above problems often have conflicting solutions. For example, idle rattle occurs when a transmission is in neutral (i.e., not connected to a load) and the transmission input shaft is connected to an engine running at or near idle speed. Under such a condition driveline torque is relatively low, and the frequencies and amplitudes of the torsionals are also relatively low. Accordingly, the torsional vibration damping mechanism must have springs of relatively low spring rate and the damper must have a relatively low breakaway torque. In-gear rattle occurs when the transmission is driving a load. Under this condition driveline torque is relatively greater and the frequencies and amplitudes of the torsionals are also relatively greater. Accordingly, the torsional vibration damping mechanism, under this condition, must have springs of relatively higher spring rate and the damper must have a relatively higher breakaway torque.
U.S. Pat. No. 4,212,380 to Billet discloses a driveline torsional vibration damping mechanism employing separate stages or assemblies for idle and in-gear conditions. Each assembly provides attenuation and damping. The assembly for idle conditions includes a set of relatively low rate springs disposed in parallel with a mechanical friction damper of relatively low breakaway torque or torque capacity. The assembly for in-gear conditions includes a set of relatively high rate springs disposed in parallel with a mechanical friction damper of relatively high breakaway torque or torque capacity. The assemblies are disposed in series and the low capacity or idle rattle assembly becomes inoperative when the transmission is driving a load.
U.S. Pat. No. 4,440,283 to Nioloux discloses a driveline torsional vibration mechanism similar to the mechanism of Billet but not having damping for the idle rattle assembly.
The Billet and Nioloux mechanisms, through improvements over prior single stage mechanisms, have added cost and complexity, and in many applications have not provided the necessary results.